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The Problem with Radar - Part 2

Not so infallible...

By Dennis Jensen

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Last week we introduced the technology of radar as it is used in police speed detection equipment. This week, we look at factors that could affect vehicle speeds measured by radar.

Cosine Effect

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This relates to what Doppler shift (see last week' article at "The Problem with Police Speed Detection Equipment - Part 1") the radar detects as opposed to the speed of the vehicle.

If the vehicle in question is moving directly towards the radar, then the Doppler shift that results will give a reading that gives the correct speed of the vehicle. If, however, the vehicle is not moving directly towards the radar unit, but rather at some angle (as would be the case if there is a bend in the road between the vehicle and the radar unit), then there will be some component of the vehicles velocity in a direction that is not directly towards the radar unit. In that case, the speed that is displayed would be lower than the speed that the vehicle is actually doing; the speed would be given by the velocity of the vehicle multiplied by the cosine of the angle between them.

It is important to note that, in the stationary mode, the error is always in favour of the vehicle in that the given reading will always be lower than the actual speed.

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In the case of mobile radar, though, the cosine effect can be to the disadvantage of the vehicle. Mobile radar works by analysing the Doppler shift of the vehicle coming towards the police vehicle, and by analysing the Doppler shift from the ground return. The ground return would give the patrol car's speed, whereas the return from the oncoming car would give the closing speed between the two vehicles. So, if the closing speed was measured at 180 km/h, and the ground return gave a reading of 100 km/h, then the ground return speed is subtracted from the combined speed to give 80 km/h, the actual speed of the vehicle approaching the police vehicle.

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Cosine error in the case of the combined speeds would result in a speed lower than actual being displayed.

If, however, the radar got a ground speed at some angle away from the direct beam direction, then the result would be a lower displayed speed for the police vehicle (say 90 km/h in our example). The result is that the displayed speed would be 90 km/h, which is higher than the speed that the target car is actually doing. This can also result if the radar unit is misaligned, and is pointing in a direction other than straight ahead.

In this diagram the angle from the police car to the target (?) will result in a lower target speed-reading. However, the angle from the police car to the surroundings (?) will result in a lower speed for the police car, giving a higher target reading.

Shadow Effect

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This relates to a problem with moving mode radar, and is once again an issue of an incorrectly displayed patrol car speed. The issue here is that the police car is following another vehicle moving in the same direction. The radar in the patrol car should be reading the ground speed, but instead has lock-on to the other vehicle travelling in the same direction, giving an incorrect patrol speed.

For example, if the police car is actually doing 80 km/h, and the other vehicle is doing 40 km/h, the patrol speed displayed would be 40 km/h. The problem is, if the oncoming car is doing a true 60 km/h, the closing doppler will still relate to a closing speed of 140 km/h. The patrol speed will be taken to be 40 km/h, so instead of a displayed speed reading of 60 km/h, the actual displayed speed will be 100 km/h.

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This is an extreme example, and unlikely to ever happen, but it does show the principle of an effect that has undoubtedly occurred in the past, and will no doubt occur again.

Double Bounce

This occurs where radar has been badly placed (such as near large street signs). The problem is that these street signs or other large surfaces reflect radar very well. The result is that the radar can reflect off the sign (or other reflector), or reflect off a car moving at a position where the police officer does not believe the vehicle with the displayed speed could be. This can result in target misidentification, with the incorrect vehicle being identified as that exceeding the speed limit.

Here it can be seen that the radar unit gives the impression that it is reading the speed of the approaching car, but the speed being measured is actually that of the car following the bus.




Interference

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There are a variety of forms of interference that can result in incorrect readings or interpretation of target identification. These can be in the form of high tension power lines, radio transmitters, airport radar and other electronic devices that use high power outputs.

There are also signs such as rotating signs, or signs that are swinging. These can result in false readings; the radar can actually detect these moving objects and display the target speed, leading to misidentification of the actual target.

In addition to man-made sources of interference, there are natural sources as well. These include things such as trees blowing in the wind, among others. With modern digital radar, however, it is highly unlikely that such natural interference will result in a reading. Rain and snow, due to the fact that the frequency that police operate is very close to the water absorption frequency, means that rains, snow and the like will significantly attenuate the radar beam, resulting in greatly reduced range performance.

Lidar

Lidar operates in the way that many people believe that radar does - the operation is pulsed, and the speed is determined by timing the time difference between returning pulses.

It must be stated that Lidar is far more likely to have a speed that is correctly attributed to a certain vehicle (even if the displayed speed is wrong). Target misidentification would be extremely rare, as the beam is coherent and has a very narrow beamwidth.

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If the laser beam is aimed and held on a certain part of the vehicle, then the speed-reading would be very accurate. There are, however, panning errors that can give incorrect readings. Take a case of a car doing 60 km/h (16.7m/s). Assume that the Lidar takes a reading in 1 second.

Now, what happens if the police officer starts the speed reading with the laser shining at the base of the windscreen, and at the end of the interrogation one second later, the beam is aimed at the front number plate, 2m ahead of the base of the windscreen? Then, even if the car was completely stationary, because the beam has been panned in this way, a reading of 2 m/s will be displayed (which is 7.2 km/h). In the case of that 60 km/h car, the displayed speed will be 67 km/h, which might result in being booked.

If the time used for the speed-reading is less, the situation is worse. For example, take the same scenario, but with the time 0.5s. In that case, the additional speed will be 2/0.5, which is 4 m/s, or 14 km/h. This type of situation was graphically illustrated a few years ago when a defence attorney panned a Lidar unit on the back wall of the courtroom and obtained a speed reading!

It must be stated that the error can, of course, also occur the other way, to the advantage of the driver.

Other Considerations...

Police are tending to use smaller and smaller tolerances for booking speeding motorists (for example, in Western Australia it is a blanket 8 km/h). The problem is that this is grossly unfair to a driver who may very well think he/she is driving at the speed limit.

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The first issue is that of actual tolerances allowed by Australian Standards (AS2898.2), where, depending on the radar, the tolerance is either 2 km/h up to 100 km/h, and then 2%, or 3 km/h and 3%. What this means is that these speeds or percentages must be taken off the detected speed.

Secondly, there is the issue of speedometer inaccuracy. Australian Design Regulations (ADR-18) allows for the speedo to be ?10% inaccurate. Think about it, the car you buy brand-new can have a speedometer that is 10% inaccurate, and it complies with standards!

Consider now what can occur with your brand-new car that under-reads by 10%. There you are, thinking that you are doing 100 km/h, but are in fact doing 110. The police detect you, and should allow 3 km/h, which means that you really should not be fined until they detect you doing at least 113 km/h. But in WA, they will book you for a DETECTED speed of 108 km/h - so much for fairness or claiming that the issue is road safety!

So, next time you are booked for speeding, don't just pay up. Consider whether the infringement you have been ticketed for has been correctly applied; if not, contest it.

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